Stops of Light Calculator

How it is calculated

Enter two amounts of light in the same unit, such as lux readings, flash power, ISO values or exposure times. The difference in stops is stops = log₂(A ÷ B). A positive number means A is brighter (more light) than B, a negative number means A is dimmer. The ratio shown is 2|stops|, which is the same as the larger value divided by the smaller one and never carries a sign. Both values must be above zero. The values have to be linear light quantities: f-numbers are not, because the light gathered goes with the square of the aperture, so f/4 and f/2.8 should not be typed here as 4 and 2.8. One stop is a doubling, two stops are 4×, three stops are 8×.

Common mistakes

• Typing f-numbers directly (4 and 2.8 gives 0.51 stops with a ratio of 1.43 ×, while the real change from f/4 to f/2.8 is one full stop). Convert to light first, for example by squaring the reciprocal of the f-number.
• Typing shutter speeds as denominators (250 and 125) and mistaking the sign: 1/125 s lets in more light than 1/250 s, but entering 125 as A and 250 as B gives −1 stop.
• Mixing units between A and B, such as lux for one and foot-candles for the other, so the ratio is wrong before the logarithm.
• Reading the ratio as a signed value; it is always 1 or more, and the sign sits only in the stops figure.
• Expecting 0 or a negative reading to work; the logarithm needs both values above zero.